1-46 



bulletin: museum of compaeatiye zoology. 



ment, because provided with a ganglion. More conclusive proof could 

 hardly be expected. 



The insect-head, then, is composed of seven somites, which are homolo- 

 gous with the first seven of decapod Crustacea. 



If the conclusions I have drawn in this paper are valid, certain radical 

 changes become necessary in the commonly accepted ideas of homology 

 among the great classes of Arthropods. These changes I submit in the 

 followins: table : — 



TABLE OF EQUIVALENT SOMITES IN THE HEAD OF AETHROPODA. 



Summary. 



The protocerebrum of Apterygota agrees with that of other insects 

 in development and structure. The ocular segments of Hexapoda and 

 decapod Crustacea, as well as the compound eyes of the two groups, are 

 homologous. 



The labrum and clypeus of insects develop from a single median 

 evagination between the procephalic lobes, and do not represent a pair 

 of appendages. The labrum of Apterygota is homologous with that of 

 other insects, as well as that of Symphyla, Diplopoda, Chilopoda, and 

 the hitrher Crustacea. 



